Messages in this thread | | | Date | Tue, 28 Oct 2008 15:58:32 +0100 | From | Jan Kratochvil <> | Subject | Re: inconsistent behavior with ptrace(TRACEME) and fork/exec |
| |
Hi,
sorry I had a race with signal()/pause() in my last post, fixed here.
On Mon, 27 Oct 2008 18:38:16 +0100, Mike Frysinger wrote: > On Mon, Oct 27, 2008 at 10:56, Jan Kratochvil wrote: > > On Wed, 19 Jul 2006 21:18:29 +0200, Mike Frysinger wrote: > >> my understanding is that if a parent forks and the child does > >> a ptrace(TRACEME) right before doing an exec(), the kernel should always > >> halt it and wait indefinitely for the parent to start ptracing it. > > > > Yes, just the parent must process the event (signal). In your testcase the > > parent finished before the signal could be delivered. If the tracer exits the > > tracee's tracing is finished and it continues freely. > > no signal should have been generated. the child should have gone > straight to the exec and waited for the parent to process it.
Every ptrace event generates SIGCHLD. vfork() frees the parent at the moment the child calls exec() or _exit(). Here the child calls exec() which both frees the parent to continue the execution AND delivers SIGCHLD with the status WIFSTOPPED WSTOPSIG(SIGTRAP) to it. I do not see a problem here.
> >> unfortunately, this behavior seems to be unreliable. > > > > Fixed the races in your code and I do not see there any problem, do you? > > The ptrace problems/testsuite is being maintained at: > > http://sourceware.org/systemtap/wiki/utrace/tests > > there is no race condition ... it's using vfork here remember ?
Yes but you said that you see the same problem even with fork():
On Wed, 19 Jul 2006 21:18:29 +0200, Mike Frysinger wrote: # also, while the attached test uses vfork(), same behavior can be observed # with fork().
> it is impossible for the parent to have executed anything after the vfork() > before the child made it into the exec() and gone to sleep
You must not rely on the vfork() specifics as vfork() can be just fork(): http://www.opengroup.org/onlinepubs/009695399/functions/vfork.html APPLICATION USAGE
I hope there is no problem with the kernel at this moment, the attached testcase works even with vfork() reliably.
Regards, Jan #define _GNU_SOURCE #include <errno.h> #include <signal.h> #include <stdint.h> #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <sys/ptrace.h> #include <sys/wait.h> #include <assert.h>
#define fail(msg, args...) \ do { \ fprintf(stderr, "FAIL:%i: " msg "\n", __LINE__, ## args); \ exit(1); \ } while (0)
static volatile int caught = 0;
static void child_exit(int sig) { int status; #if 0 /* Correct behavior. */ printf("failure, child exited with %i: %s\n", sig, strsignal(sig)); #else assert(sig == SIGCHLD); #endif printf("wait() = %i\n", wait(&status)); printf("status = 0x%x\n", status); printf("\tWIFEXITED = %i\n", WIFEXITED(status)); printf("\tWEXITSTATUS = %i\n", WEXITSTATUS(status)); printf("\tWIFSIGNALED = %i\n", WIFSIGNALED(status)); printf("\tWTERMSIG = %i (%s)\n", WTERMSIG(status), strsignal(WTERMSIG(status))); /* WIFSTOPPED happens. */ printf("\tWIFSTOPPED = %i\n", WIFSTOPPED(status)); /* SIGTRAP happens. */ printf("\tWSTOPSIG = %i (%s)\n", WSTOPSIG(status), strsignal(WSTOPSIG(status))); #if 0 /* We can continue. Just calling printf() from a signal handler is not correct. */ exit(1); #else caught = 1; #endif }
int main(int argc, char *argv[]) { long pret; pid_t pid;
/* child process ... shouldnt be executed, but just in case ... */ if (argc > 1 && !strcmp(argv[1], "child")) #if 0 /* Parent did not kill us, after its child_exit() messages we should get here. */ fail("kernel should have halted me..."); #else { puts ("child exiting"); exit (0); } #endif
/* Setup the handler already here as after vfork() there would be a race setting it up before we get signalled. */ signal(SIGCHLD, child_exit);
/* vfork() child must not call ptrace(). */ pid = vfork(); if (pid == -1) fail("vfork() didnt work: %m"); else if (!pid) { /* do the child stuff here */ errno = 0; pret = ptrace(PTRACE_TRACEME, 0, NULL, NULL); if (pret && errno) fail("ptrace(PTRACE_TRACEME) = %li: %m", pret);
int eret = execlp(argv[0], argv[0], "child", NULL); fail("execlp() = %i", eret); } /* do the parent stuff here */
/* We cannot pause() here as the signal already could occur, we would have to SIG_BLOCK SIGCHLD and sigsuspend() here. */ while (!caught);
errno = 0; pret = ptrace(PTRACE_PEEKUSER, pid, NULL, NULL); if (pret && errno) fail("ptrace(PTRACE_PEEKUSER, %i) = %li: %m", pid, pret);
puts("SUCCESS! :D");
return 0; }
| |